ath9k: fix panic while cleaning up virtaul wifis

num_sec_wiphy means max secondary wifis that the driver can accomudate.
So cancelling wiphy work should be based on the presence of
secondary wifis.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>